Frequency responsive switching circuit

ABSTRACT

A frequency responsive switching circuit particularly suitable for use with apparatus for limiting the position of the throttle of an internal combustion engine at a predetermined engine speed. The circuit includes first and second retriggerable monostable multivibrators, each having a different timing circuit, which cooperate with one another to produce a switched output signal when the frequency level of the circuit input signal has reached a predetermined magnitude. The frequency responsive switching circuit is designed for use in a motor vehicle environment and may derive its input signal from the primary winding of an ignition coil conventionally included in motor vehicles.

United States Patent Talmage et al.

1 1 Oct. 21, 1975 FREQUENCY RESPONSIVE SWITCHING Primary ExaminerRobertK. Schaefer CIRCUIT Assistant Examiner-M. Ginsburg [75] Inventors:Dennis Talmage Plymouth; Paul Attorney, Agem, 0r Firm-Robert W. Brown;Keith L.

n. Stokes, Westland, both of Mich. Zerschlmg [73] Assignee: Ford MotorCompany, Dearborn, 57] ABSTRACT A frequency responsive switching circuitparticularly [22] Fil d; J n, 8, 1975 suitable for use with apparatusfor limiting the position of the throttle of an internal combustionengine at a [21] Appl' 539571 predetermined engine speed. The circuitincludes first and second retriggerable monostable multivibrators, [52]US. Cl. 307/129; 123/102 each having a different timing circuit, which p51 Int. Cl. 110111 35 06 are with one another to produce a SwitchedOutput l [58] Field of Search 307/129, 125, 232, 233, he! when thefrequency level of the Circuit input Signal 307/234; 123/97 B, 102 hasreached a predetermined magnitude. The frequency responsive switchingcircuit is designed for use 56 References Cit d in a motor vehicleenvironment and ma derive its l l y UNITED STATES PATENTS input signalfrom the primary winding of an ignition 3 645 24] 2,1972 Hunmnger123/102 coil conventionally included in motor vehicles.

10 Claims, 2 Drawing Figures 1 914 IP/ L t/ I! W M 1 PI 71 I jg, P! A!'/;a //W/ll mm i4, A

A! 0d ,w //l 2P1 P45: ll 6 M I71 2! 14 l\ 4; l I ///0 7/161 M/a 1\ T F6m w /:1 'Zl o M F 7m El]! 75 Pa'fi'zZu/w' 4 i In 14404 1 E1411 #1 '41 #4I /0/(n Wm :1 //4 Sheet 1 of 2 3,914,619

US. Patent Oct. 21, 1975 U.S. Patent Oct. 21, 1975 Sheet20f2 3,914,619

This invention relates to a frequency responsive switching circuit. Moreparticularly, the invention relates to a frequency responsive switchingcircuit which may be used in a motor vehicle to control the setting ofanactuator that limits the position of the engine throttle such that thethrottle cannot be fully closed, but rather must be at least partiallyopen, when the engine speed is above a predetermined level.

In certain vehicle applications, it is desirable to maintain thethrottle of an internal combustion engine in a partially open conditionat engine speeds above a predetermined level for'the purpose ofminimizing undesirable engine exhaust emissions. Maintaining thethrottle in a partially open position may be especially desirable duringengine deceleration. Apparatus which limits throttle position to apartially open condition when engine speeds are above a predeterminedlevel are in commercial use and thus are known in the prior art.

SUMMARY OF THE INVENTION The present invention provides a novelelectronic circuit particularly suitable for use in motor vehicles tolimitthrottle position in the manner described above. The circuitutilizes ret'riggerable monostable multivibrators to achieve a switchingfunction at a'predetermined input frequency magnitude. Separate timingcircuits are provided for each of two retriggerable monostablemultivibrators in the circuit, and the multivibrators are interconnectedwith thetiming circuits in a manner which provides hysteresis in theswitching operation. This hysteresis causes the frequency responsiveswitching circuit to produce an output signal in a first condition at afirst predetermined frequency level of the input signal, as thefrequency of that input signal increases, and to produce switching ofits output to a second condition at a second predetermined frequencylower than the first predetermined frequency as the frequency of theinput signal decreases. This hysteresis provides stability in theswitching function of the circuit. 1

Preferably, the two retriggerable monostable multivibrators are includedin a single integrated circuit'package preferably of the commerciallyavailable type 556 and it is specifically preferred that'they be aSignetics Corporation type NE556 dual timer. i

The invention may be better understood by reference to the detaileddescription which follows and to the drawings. I

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view ofapparatus for limiting or setting the position of the throttle in theengine of a motor vehicle in response to an electrical signal;

and

FIG. 2 is a schematic diagram of a frequency responsive switchingcircuit suitable for use in controlling the apparatus illustrated inFIG. 1.

v DETAILED DESCRIPTION With reference now to the drawings, wherein likenumerals refer to like parts in the two views and wherein componentvalues and type numbers for circuit components are given by way ofexample and not limitation,

there is shown in FIG. 1 a diagrammatic view of apparatus for limitingor setting the position of the throttle of an internal combustionengine. The engine includes an intake manifold 10 on which is positioneda carbure- 1 tor 12. The carburetor 12 has a throttle plate 14 shown inits fully closed position. The throttle plate 14 is mounted for pivotalmovement about its mounting axis in the conventional manner. A link 16is secured by a screw 18 to one end of the shaft about which thethrottle plate 14 rotates. A linking wire 20 is pivotably connected tothe link 16 at 22. The opposite end 24 of the linking wire is formed ina manner which permits it 'to be inserted in and move within a slot 26in the movable arm 28 of a vacuum 30.

The vacuum motor 30 includes a flexible diaphram 32 which divides thevacuum motor housing 34 into chambers 36 and 38. Atmospheric air is freeto enter the chamber 36 through a slot in the housing 34 through whichthe movable arm 28 passes. The diaphram 32 is urged in the direction ofthe chamber 36 by a compression spring 40 located in the chamber 38.When vaccum is applied to the chamber 38 through an inlet passage 42communicating therewith, the diaphram 32 is drawn to the left as viewedin FIG. against the force of the compression spring 40. The leftwardtravel of the diaphram 32 is limited by an adjustable screw 44. When thediaphram 32 moves, the movable arm 28 attached thereto also moves towardthe left as viewed in FIG. 1.

The vacuum motor 30 is secured to the housing of the carburetor 12 by amounting bracket 46. When the vacuum signal is applied to the vacuummotor inlet passage 42, the movable arm 28 is pulled to the left andpulls the linking wire 20 with it which, in turn,.pivots the link 16 andthe throttle plate 14 about its axis. This opens the throttle plate 14to a minimum partially open position. The amount that the throttle plateis opened is determined by the setting of the adjustable screw 44 whichlimits the travel of the vacuum motor diaphram 32. Of course, thethrottle plate 14 may .be opened to a greater extent than that producedby actuation of the vacuum motor 30 because the linking wire 20 isfreely movable within the slot 26.

The vacuum motor 30 is controlled by a solenoid valve 48, a transistorQ4 and, if desired, a vacuum switch 50 responsive to the vacuum level inthe engine intake manifold 10.

Electrical terminals 52, 54, and 56 are terminals of the electroniccircuit shown in FIG. 2. Lead wire 58 connected to terminal 52 suppliesDC voltage through an electrical connector 60 to one lead of anelectrical coil 62 controlling the operation of the solenoid valve 48.The other lead of the electrical coil 62 passes into the electricalconnector 60 and is connected by a lead wire 64 to the terminal 54 ofthe electronic circuit in FIG. 2, which includes the transistor 04.Transistor O4 is a power transistor the collector of which is connectedto the terminal 54 and'the emitter of which is connected to the terminal56.

A lead wire 66 connects the terminal 56 of the transistor O4 to aterminal 68 of the vacuum switch 50. The vacuum switch 50 includes ahousing formed from cupshaped portions 70 and 72 between which aflexible diaphram 74 is clamped. The chamber 76 formed between thediaphram 74 and the housing portion 72 is in communication withatmospheric pressure through an opening 78. A chamber 80 is formedbetween the diaphram 74 and the housing portion 70, and this chamber 80communicates, through tubing 82, T- connection 84, and tubing 86, withthe interior of the intake manifold 10. When the level of vacuum withinthe intake manifold is sufficiently high, a movable contact arm 88attached to terminal 68 of the vacuum switch 50 is urged, by thediaphram 74 and against the force of a compression spring 90, intoelectrical contact with an electrode 92 electrically connected with agrounded terminal 94 of the vacuum switch 50. The vacuum switch 50 maybe calibrated to achieve electrical continuity between its terminals 68and 94 at, for example, vacuum levels in the chamber 80 of about 19inches of mercury.

The solenoid valve 48 has a movable plunger 96 which has the positionshown when its electrical coil 62 is de-energized. In this condition,atmospheric air freely enters the solenoid valve through a passage 98.This passage has a small orifice 100. Air may flow through this orifice,alongside the movable plunger 96, which isof noncircular cross-section,and into an outlet passage 102 which communicates via tubing 103 withthe chamber 38 in the vacuum motor30.

v When the electrical coil 62 of the solenoid valve 48 is energized, themovable plunger 96 is pulled upwardly as viewed in FIG. 1 to block theorifice 100 and to permit the outlet passage 102 to communicate with apassage 104. A porous metal filter 106 provides an atmospheric bleed tothe passage 104 to assist the operation ofthe solenoid valve 48. Thepassage 104 is in' communication, via tubing 108, T-connection 84 andtubing 86, with the interior of the intake manifold 10. Thus, when thesolonoid valve 48 is energized, manifold vacuum'is applied to chamber 38of the vacuum motor 30 via passage 104 and valve outlet passageconnected by tubing 103 to the chamber 38. Thus, whenever the coil ofthe solenoid valve 48 is energized, the vacuum motor 30 is actuated toprevent the throttle plate 14 from being fully closed, that is, tomaintain it at least at a partially open position.

The function of the apparatus shown in FIG. 1 is to prevent the throttleplate 14 from becoming fully closed when engine speed is above apredetermined level, for example, 1850 rpm, particularly when the engineis decelerating. The vacuum switch 50 senses engine decelerationsbecause the level of manifold vacuum increases under such conditions.The collectoremitter output circuit of the transistor O4 is connected inseries with the vacuum switch 50 and in series with the electrical coil62 of the solenoid valve 48. The transistor Q4 output circuit is fullyconductive whenever the engine speed is above a predetermined level and,if the manifold vacuum is sufficiently high to close the vacuum switch50, the solenoid valve 48 is energized to actuate the vacuum motor 30and to limit the position of the throttle plate 14.

Of particular importance to the present invention is the electroniccircuit shown in FIG. 2 of which the output transistor Q4 forms a part.The circuit in FIG. 2 includes a frequency responsive circuit, generallydesignated by the numeral 110, which has input terminals 112, 114, and116, the voltage supply terminal 52 and output terminals 54 and 56. Asource of DC electrical energy 118, which preferably is a conventionalvehicle DC storage battery, has its negative terminal connected by alead 120 to ground at 122 and to the input terminal 116 of the frequencyresponsive circuit 110. Thus,

a lead 124 in the circuit 110 is a ground voltage supply lead. Thepositive terminal of the DC source 118 is connected by a lead 126 to aconventional vehicle ignition switch 128. The opposite terminal of theignition switch 128 is connected by a lead 130 to the input terminal 112of the frequency responsive circuit 110. This provides the positivevoltage supply forthe circuit 110.

Terminal 112 of the circuit 110 is connected through a resistor R1 to apositive voltage supply lead 132. This lead is maintained atsubstantially the potential on supply lead 130 connected to terminal112. A zener diode D1 has its cathode connected to the voltage supplylead 132 and its anode is connected to ground. A capacitor C1 isconnected in parallel with the zener diode D1. The zener diode protectsthe frequency responsive circuit 110against voltage transients whichmight occur at terminal 112. The capacitor C1 acts as a high frequencypower supply filter.

lnput terminal 114 of the frequency responsive circuit 110 is connectedby a lead 134 to one terminal of the primary winding 136 of the ignitioncoil conventionally included in a vehicle ignition system. The secondarywinding 138 of the ignition coil is connected to the ignition systemdistributor. A balast resistor 140 is connected between the voltagesupply lead 130 and the terminal of the ignition coil primary windingconnected to input terminal 114. Preferably, the impedance of the balastresistor matches the impedance of the primary winding 136. Conventionalbreaker contacts 142, or the electronic equivalent, open and close intimed relation to engine operation to initiate and interrupt currentflow in the ignition coil primary winding 136 in the conventionalmanner. This produces a periodic waveform on lead 134 connected to inputterminal 114. With a twelve-volt DC source 118, the periodic signal atinput terminal 114 varies from about a maximum of 12 volts, when thebreaker contacts 142 are open, to a minimum of about 6 volts when thebreaker contacts 142 are closed during the ignition system dwell time.

The frequency responsive switching circuit 110 includes a SigneticsCorporation type NE556 dual timer. This dual timer is commerciallyavailable and is depicted in H6. 2 as circuit elements 144 and 146, eachof these being designated as one-half of the type NE556 dual timer. Thepin numbers indicated in the circuit elements 144 and 146 areconventional for the commercially available dual timer. Eachof thecircuit elements 144 and 146 is a retriggerable monostable multivibratorand hereinafter isreferred to as such.

With respect to multivibrator 144, its ground pin 7 is connected by alead 148 to the circuit ground lead 124. Its voltage supply pin 14 isconnected by a lead 150 to the voltage supply lead 132. Pin 1 is thedischarge terminal of multivibrator 144 and is connected to its pin 2,which is the threshold terminal for its external timing circuit. Pin 4is the reset input of multivibrator 144 and pin 6 is its trigger input.Pin 5 is the output terminal for multivibrator 144 and is connectedthrough a blocking diode D4 and by a lead 152 to the trigger input pin 8of the multivibrator 146.

Pin 13 is the discharge terminal for the external timing circuit ofmultivibrator 146 and is connected with pin 12, which is the thresholdterminal. Pin 10 of multivibrator 146 is its voltage supply terminal andis connected by a lead 154 to the voltage supply lead 132. Pin

9 is the output terminal of multivibrator 146.

The output pin 9 of multivibrator 146 is connected through a currentlimiting resistor R10 to the base of a transistor Q3 whose emitter isconnected to ground supply lead 124. The collector of the transistor Q3is connected through a current limiting resistor R11 to a voltage supplylead 156 connected to voltage input terminal 112; it should be notedthat supply lead 156 provides the positive voltage supply for thesolenoid valve 48 connected to terminal 52. The collector of thetransistor O3 is connected by a lead 158 to the base of the transistorQ4. Multivibrator 146 switches its pin 9 output between low and highvoltage levels. At its low voltage level (approximately 0.1 volt), thetransistor O3 is nonconductive which places its collector at the voltageon supply lead 156 and renders the transistor Q4 conductive to energizethe coil 62 of the solenoid valve 48. When the pin 9 output voltage isat its highest level (approximately 1.6 volts below the voltage supplyon lead 132), the transistor Q3 is conductive and the transistor O4 isnon-conductive resulting in de-energization of solenoid valve 48.Transistors Q3 and Q4 provide power amplification of the signal onoutput pin 9 of multivibrator 146.

The alternating input signal at terminal 114 of the frequency responsiveswitching circuit 110 is applied through a current limiting resistor R2to the base of a transistor Q1 whose emitter is connected by a lead 160to the voltage supply lead 132 and whose collector is connected througha current limiting resistor R3 to the ground lead 124. A diode D2 hasits anode connected to the base of transistor Q1 and has its cathodeconnected to voltage supply lead 132. Diode D2 protects the base-emitterjunction of the transistor Q1 against transients. A capacitor C2 isconnected in parallel with the diode D2. Capacitor C2 acts as a highfrequency filter for the voltage oscillations normally associated withthe primary side of a vehicle ignition system. Since the alternatingsignal applied at the input terminal 114 is referenced to the positivevoltage supply of the circuit, transistor Q1 and resistors R2 and R3 areincorporated to provide an alternating signal at the collector of Q1that is referenced to ground potential and that has increased rise andfall rates as compared to the input signal.

When the input signal at terminal 114 is a high voltage levelcorresponding to open breaker contacts 142, the transistor 01 isnonconductive and the collector of transistor O1 is at ground potential.When the breaker contacts 142 close to initiate the ignition systemdwell time, the input signal applied to terminal 114 falls to about 6volts in a l2-volt vehicle ignition system, and the transistor O1 isrendered conductive placing its collector at substantially supply line132 voltage. The resulting waveform at the collector of O1 is indicatedat 162 where the negative-going edges 164 of this alternating waveformcorrespond to the end of the ignition system dwell time occurring uponthe opening of the breaker contacts 142. This alternating waveform 162is applied by a lead 166 to the trigger input pin 6 of the multivibrator144.

Preferably, equal-value resistors R4 and R5 are connected in series withone another between the voltage supply leads 132 and 124. This forms avoltage divider that provides a quiescent operating point, equal toonehalf the voltage on supply lead 132, on a lead 168 connecting thejunction of the resistors R4 and R5 to the reset input pin 4 of themultivibrator 144. A coupling capacitor C3 is connected between thecollector of the transistor Q1 and the junction between resistors R4 andR5. Capacitor C3 modifies the waveform 162 on the collector of thetransistor 01 to produce a waveform 4 170 at the reset pin 4 ofmultivibrator 144. This waveform 170 consists of voltage spikes whichare negative and positive going with respect to the quiescent voltagelevel established at the junction between the resistors R4 and R5. Thenegative-going voltage spikes 172 correspond to the negative-going edges164 of the waveform 162, whereas the positive-going spikes correspond tothe positive-going edges of waveform 162. A clamping diode D3 has itsanode connected to reset pin 4 and its cathode connected to voltagesupply lead 132. This diode limits the voltage at pin 4 to one diodevoltage drop above the potential of voltage supply lead 132.

The external timing circuit for the multivibrator 144 is an RC circuit.This circuit includes a variable resistance P1, a resistor R6 and acapacitor C4. These elements are connected in series with one anotherand between the voltage supply leads 132 and 124. Also, a resistor R12has one of its terminals connected to the junction 174 formed betweenthe resistor R6 and the capacitor C4 and has its other terminalconnected to the cathode of a blocking diode D5 whose anode is connectedto the output pin 9 of the multivibrator 146. When the output voltage atpin 9 of the multivibrator 146 is at its high voltage level, the diodeD5 is forwardbiased placing theresistor R12 in parallel withseriesconnected resistances P1 and R6. Thus, under this circumstance,the resistor R12 forms a part of the timing circuit for multivibrator144. As is further explained hereinafter, this provides hysteresis inthe frequencyswitching function of the circuit 110 because it affectsthe charging rate of the capacitor C4 and because the voltage acrossthis capacitor is applied to pins 1 and 2 of multivibrator 144.

The multivibrator 144 has an internal transistor whose collector-emitteroutput circuit is connected between its discharge pin 1 and ground pin7. This internal transistor is conductive when the voltage at output pin5 is at its low level. Conduction of the internal transistor dischargesthe capacitor C4 to ground. Preferably, resistor R6 is of the carbonfilm type and capacitor C4 is of the r'nylar type. These components thenhave positive and negative temperature coefficients, respectively, andtheir RC time constant is substantially independent of temperature inthe range from about 40F.

to F.

The output at pin 5 of the multivibrator 144 is applied through blockingdiode D4 to lead 152. A filter network comprises a resistor R7 havingone of its leads connected to the cathode of the diode D4 and having itsother lead connected to ground voltage supply lead 124. A capacitor C5,connected in parallel with the resistor R7, also forms a part of thefilter network.

A timing circuit for the multivibrator 146 includes a resistor R9 and acapacitor C6 connected in series between the voltage supply leads 132and 124. The junction between the resistor R9 and the capacitor C6 isconnected to interconnected threshold pin 12 and discharge pin 13 ofmultivibrator 146. The emitter of a transistor O2 is connected to thisjunction and the collector of the transistor O2 is connected to groundsupply lead 124. The base of the transistor O2 is connected through aresistor R8 to the lead 152. The transistor Q2 when conductive in itsemitter-collector output circuit discharges the capacitor C6.

In the operation of the frequency responsive switching circuit 110, theretriggerable monostable multivibrators 144 and 146 are triggered byfalling waveforms applied to their respective trigger inputs 6 and 8.Also, the trigger pulse applied to the trigger input of themultivibrator 144 renders its internal discharging transistor fnonconductive and initiates the charging of the timing capacitor C4. Theinternal circuitry of the multivibramines the duration of the highvoltage level pulse at output pin of multivibrator 144. The timingcircuit components P1, R6 and C4 determine the maximum duration of thishigh level pulse width according to the equation j 1/1.1(R6+Pl )C4 wherefl, is a predetermined frequency of response for the multivibrator 144which is equal to the reciprocal of the pulse duration at its output pin5. WI-Ien the output pin 9 of multivibrator 146 is at a high voltagelevel to place the resistor R12 in parallel with resistances P1 and R6,the capacitor C4 charges more rapidly and the duration of the outputpulse at pin 5 of multivibrator 144 is decreased. The reciprocal of thispulse duration corresponds to a frequency f defined by the equation:

R6 Pl R12 1.l(R6 P1)(Rl2)(C4) When the voltage level on output pin 5 ofmultivibrator 144 falls to ground potential, the blocking diode D4 isreverse-biased and the base of the transistor 02 is coupled to groundthrough resistor R8 and the filter network including resistor R7 andcapacitor C5. This 7 renders the transistor Q2 conductive to dischargethe capacitor C6. A negative-going signal appears on the lead 152 totrigger the monostable multivibrator 146. Its output pin 9 then goes toa high potential level.

As was previously mentioned, the timing circuit for the multivibrator146 includes resistor R9 and capacitor C6. The capacitor C6 begins tocharge when the multivibrator 146 is triggered. The timing circuitprovides an output pulse duration on pin 9 of the multivibrator'146 thereciprocal of which corresponds to a frequency f l/1.1(R9)(C6) that ismuch lowerthan either of the frequenciesf or f Let it be assumed thatthe frequency of the input sig-' nal at terminal 114, and of the shapedcorresponding waveform 162, is less than )1 In such case, consecutivenegative-going edges 164 of the input waveform 162 are spaced too farapart in time to retrigger the multivibrator 144 before it has completedits timing cycle. In other words, during each period of the inputwaveform 162, the capacitor C4 is charged to two-thirds of the voltageon supply line 132, the internal discharging transistor in themultivibrator 144 is rendered conduc I tive, and capacitor 4 isdischarged. Thus, the output at pin 5 of multivibrator 144 has thewaveform shown at 180. The corresponding signal on pins 1 and 2 ofmultivibrator 144 is shown at 186.

Each of the negative-going edges of the waveform 180 are coupled bydiode D4 and'lead 152 to the trigger input of the multivibrator 146.Once the multivibrator 146- is triggered to produce a high voltage levelat its output pin 9, this high voltage level is maintained because eachnegative-going signal on lead 152 retriggers multivibrator 146 beforecapacitor C6 has had sufficient time to charge to a potential equal totwo-thirds of the potential on supply lead 132. Also, each of thesenegative-going edges on lead 152 renders the transistor Q2 conductiveand discharges the capacitor C6. In

view of this, the output signal on pin 9 of multivibrator 146 ismaintained at its high voltage level, transistor Q3 is conductive, andtransistor O4 is nonconductive maintaining solenoid valve 48deenergized.

With the voltage at pin 9 of multivibrator 146 high, the resistor R12 isconnected in parallel with the resistances P1 and R6 so that thefrequency setting for multivibrator 144 is the frequency f As enginespeed increases, the frequency of the input signal at terminal 114increases as does the frequency of the waveform 162. .When the inputfrequency reaches the predetermined frequency f the multivibrator 144 nolonger is able to complete its timing cycle because it is retriggered bythe negative-going pulses 172 and negativegoing edges 164 applied,respectively, to its reset pin 4 and its trigger pin 6. Each of thepulses 172 applied to reset pin a discharges the capacitor C4. Theoutput waveform 182 illustrates the signal appearing at pin 5 when thefrequency of the input signal 162 exceeds the predetermined frequency fWaveform 188 shows the signal on multivibrator pins 1 and 2.

From the waveform 182, it may be seen that the output voltage on pin 5ismaintained in a high voltage level condition except for short-durationnegativegoing pulses 184. The pulses 184 unfortunately are the result ofapplying the negative going spikes 172 to the reset pin 4 and are ofequal duration.

The filter network comprising resistor R7 and capacitor C5 is designedeffectively to remove the negative going pulses 184 at pin 5 from thewaveform appearing on lead 152. When the negative-going pulses 184occur, the diode D4 is reverse-biased and the capacitor C5, having beencharged when the waveform 182 was at its high voltage level, candischarge only through the resistor R7. The time constant (R7)(C5) issufficiently large to prevent the negative-going pulses 184 fromtriggering multivibrator 146. As a result the signal on lead 152 ismaintained at a'substantially constant high and R6 in the timing circuitfor multivibrator 144. Thus, the multivibrator 144 frequency is set at fa lower frequency than the frequency f This provides hysteresis in thecircuit such that the frequency of the input signal applied to terminal114 must fall below the frequencyf in order to cause the pin 9 output ofmultivibrator 146 once again to attain the high voltage level.

required to de-energi ze solenoid valve 48.

It should be noted that the multivibrator 144 is triggered on eachnegative-going edge 164 of the input waveform 162 and that its outputpulse duration is independent of the duty cycle of the waveform 162.Output pulse duration is determined exclusively by the external timingcircuit connected to pins 1 and 2 of multivibrator 144.

With the component values indicated in FIG. 2, the frequency f equalsabout 123 Hz and the frequency f, is about 125 Hz. These valuescorrespond, respectively, in a four cycle, eight-cylinder engineapplication, to 1845 and 1875 engine rpm, a difference between them ofabout 30 rpm. Of course, the ignition system of an engine having asmaller number of cylinders produces a lower frequency input signal atterminal 114. The timing circuits for the multivibrators 144 and 146necessarily would have to be adjusted accordingly to achieve switchingoperation of the circuit 110 at corresponding engine rpm levels.

Based upon the foregoing description of the invention, what is claimedis:

l. A frequency responsive switching circuit, which comprises, incombination:

an integrated circuit dual timer comprising first and secondretriggerable monostable multivibrators;

circuit means coupling the output of said first multivibrator to theinput of said second multivibrator;

timing circuits for each of said multivibrators, the timing circuit forsaid first multivibrator being set to produce an output pulse durationless than the output pulse duration set by the timing circuit for saidsecond multivibrator; and

circuit means for modifying the timing characteristic of said timingcircuit for said first multivibrator in response to the occurrence of asignal condition at the output of said second multivibrator. I

2. A frequency responsive switching circuit according to claim 1 whereinsaid first multivibrator includes a trigger input and a reset input andwherein said frequency responsive switching circuit further includes acapacitor for coupling a signal applied to said trigger input to saidreset input.

3. A frequency responsive switching circuit, which comprises, incombination:

first and second retriggerable monostable multivibrators;

a first timing circuit, associated with said first multivibrator, saidfirst timing circuit determining the maximum frequency of an inputsignal applied to said first multivibrator that will permit said firstmultivibrator to complete its timing cycle before being retriggered;

a second timing circuit, associated with said second multivibrator, saidsecond timing circuit determining the maximum frequency of an inputsignal applied to said second multivibrator that will permit said secondmultivibrator to complete its timing cycle before being retriggered,said maximum frequency determined by said second timing circuit beinglower than said maximum frequency determined by said first timingcircuit; and

circuit means coupling said first timing circuit to the output of saidsecond multivibrator, said maximum frequency determined by said firsttiming circuit being capable of being modified by a change in voltagelevel at said output of said second multivibrator.

4. A frequency responsive switching circuit, which comprises, incombination:

first and second retriggerable monostable multivibrators, each of saidmultivibrators having a trigger input and an output, said multivibratorsbeing capable of producing pulse output signals in response to triggerinput signals, the output of said first multivibrator being coupled tothe input of said second multivibrator,

first timing circuit means, associated with said first multivibrator,for determining the time duration of pulses occurring at said output ofsaid first multivibrator,

second timing circuit means, associated with said second multivibrator,for determining the time duration of pulses occurring atsaid output ofsaid second multivibrator;

circuit means for resetting said first timing circuit means as afunction of the frequency of the signal applied to said trigger input ofsaid first multivibrator;

circuit means for resetting said second timing circuit means as afunction of the frequency of the signal applied to said trigger input ofsaid second multivibrator; and

circuit means, coupled to said output of said second multivibrator andto said firsttiming circuit means, for modifying the timingcharacteristic of said first timing circuit means in response to achange in the output of said second multivibrator.

5. A frequency responsive switching circuit according to claim 4 whichfurther includes circuit means for retriggering said first multivibratorduring each period of an alternating signal applied to said triggerinput of said first multivibrator.

6. A frequency responsive switching circuit according to claim 5 whichfurther includes circuit means for retriggering said secondmultivibrator in response to a change in voltage level at said output ofsaid first multivibrator.

7. A frequency responsive switching circuit according to claim 5 whereinsaid circuit means for retriggering said first multivibrator includes avoltage divider, said first multivibrator having a reset input coupledto said voltage divider, and wherein said frequency responsive switchingcircuit further includes a capacitor coupled between said voltagedivider and said trigger input of said first multivibrator.

8. A frequency responsive switching circuit according to claim 5 whereinsaid first multivibrator has a reset input and wherein said frequencyresponsive switching circuit further includes circuit means forestablishing a quiescent voltage level at said reset input and acapacitor coupled to said reset input and to said trigger input of saidfirst multivibrator to retrigger said first multivibrator at a frequencycorresponding to the frequency of the signal applied to said triggerinput of said first multivibrator.

9. A frequency responsive switching circuit according to claim 5 whichfurther includes circuit means for rendering the output response of saidfirst multivibrator independent of the duty cycle of the signal appliedto said trigger input of said first multivibrator.

10. A frequency responsive switching circuit according to claim 5wherein said first timing circuit means comprises an RC timing circuitincluding a carbon film resistor and a mylar capacitor, said resistorand capacitor having temperature characteristics rendering said firsttiming circuit means substantially independent of temperature in therange from about 40F. to F.

1. A frequency responsive switching circuit, which comprises, incombination: an integrated circuit dual timer comprising first andsecond retriggerable monostable multivibrators; circuit means couplingthe output of said first multivibrator to the input of said secondmultivibrator; timing circuits for each of said multivibrators, thetiming circuit for said first multivibrator being set to produce anoutput pulse duration less than the output pulse duration set by thetiming circuit for said second multivibrator; and circuit means formodifying the timing characteristic of said timing circuit for saidfirst multivibrator in response to the occurrence of a signal conditionat the output of said second multivibrator.
 2. A frequency responsiveswitching circuit according to claim 1 wherein said first multivibratorincludes a trigger input and a reset input and wherein said frequencyresponsive switching circuit further includes a capacitor for coupling asignal applied to said trigger input to said reset input.
 3. A frequencyresponsive switching circuit, which comprises, in combination: first andsecond retriggerable monostable multivibrators; a first timing circuit,associated with said first multivibrator, said first timing circuitdetermining the maximum frequency of an input signal applied to saidfirst multivibrator that will permit said first multivibrator tocomplete its timing cycle before being retriggered; a second timingcircuit, associated with said second multivibrator, said second timingcircuit determining the maximum Frequency of an input signal applied tosaid second multivibrator that will permit said second multivibrator tocomplete its timing cycle before being retriggered, said maximumfrequency determined by said second timing circuit being lower than saidmaximum frequency determined by said first timing circuit; and circuitmeans coupling said first timing circuit to the output of said secondmultivibrator, said maximum frequency determined by said first timingcircuit being capable of being modified by a change in voltage level atsaid output of said second multivibrator.
 4. A frequency responsiveswitching circuit, which comprises, in combination: first and secondretriggerable monostable multivibrators, each of said multivibratorshaving a trigger input and an output, said multivibrators being capableof producing pulse output signals in response to trigger input signals,the output of said first multivibrator being coupled to the input ofsaid second multivibrator, first timing circuit means, associated withsaid first multivibrator, for determining the time duration of pulsesoccurring at said output of said first multivibrator; second timingcircuit means, associated with said second multivibrator, fordetermining the time duration of pulses occurring at said output of saidsecond multivibrator; circuit means for resetting said first timingcircuit means as a function of the frequency of the signal applied tosaid trigger input of said first multivibrator; circuit means forresetting said second timing circuit means as a function of thefrequency of the signal applied to said trigger input of said secondmultivibrator; and circuit means, coupled to said output of said secondmultivibrator and to said first timing circuit means, for modifying thetiming characteristic of said first timing circuit means in response toa change in the output of said second multivibrator.
 5. A frequencyresponsive switching circuit according to claim 4 which further includescircuit means for retriggering said first multivibrator during eachperiod of an alternating signal applied to said trigger input of saidfirst multivibrator.
 6. A frequency responsive switching circuitaccording to claim 5 which further includes circuit means forretriggering said second multivibrator in response to a change involtage level at said output of said first multivibrator.
 7. A frequencyresponsive switching circuit according to claim 5 wherein said circuitmeans for retriggering said first multivibrator includes a voltagedivider, said first multivibrator having a reset input coupled to saidvoltage divider, and wherein said frequency responsive switching circuitfurther includes a capacitor coupled between said voltage divider andsaid trigger input of said first multivibrator.
 8. A frequencyresponsive switching circuit according to claim 5 wherein said firstmultivibrator has a reset input and wherein said frequency responsiveswitching circuit further includes circuit means for establishing aquiescent voltage level at said reset input and a capacitor coupled tosaid reset input and to said trigger input of said first multivibratorto retrigger said first multivibrator at a frequency corresponding tothe frequency of the signal applied to said trigger input of said firstmultivibrator.
 9. A frequency responsive switching circuit according toclaim 5 which further includes circuit means for rendering the outputresponse of said first multivibrator independent of the duty cycle ofthe signal applied to said trigger input of said first multivibrator.10. A frequency responsive switching circuit according to claim 5wherein said first timing circuit means comprises an RC timing circuitincluding a carbon film resistor and a mylar capacitor, said resistorand capacitor having temperature characteristics rendering said firsttiming circuit means substantially independent of temperature in therange from about -40*F. to 185*F.